Preheat the oven to 350 degrees F. Mix the flour, baking soda, pumpkin pie spice, and salt together in a medium bowl.
Beat the butter and both sugars together with an electric mixer. Then add the egg, vanilla, and molasses and beat again to combine. Scrape the bowl and add the flour mixture a little at a time. Once the dough has come together, divide it onto two pieces of plastic wrap. Wrap the dough into two disks and refrigerate for at least 30 minutes.
Remove one disk from the fridge. Flour a work surface and roll it out to between 1/8- and 1/4- inch thick. Using a 2-inch cookie cutter, cut out the cookies as close together as possible. Use a spatula to transfer the cookies to parchment paper-lined cookie sheets. The dough is fragile, but remember fragile dough equals light tender cookies. Gather the leftover dough and repeat. Try to get as many cookies cut out of two rolls on each disk. The more you gather and re-roll, the tougher the dough gets!
Bake for 10-15 minutes. Allow the cookies to cool. Raise the oven rack to the highest position and turn on the broiler. Using a piping bag, or a freezer bag with the corner cut off, pipe the marshmallow fluff over each cookie. The goal is to get a smooth layer about 1/3 the thickness of the cookie. If you add too little it will get bumpy when you toast it, and if you add too much, it will run off. I used about a tablespoon per cookie.
The marshmallow fluff will smooth out after a minute or two. Toast the cookies under the broiler for about 30 seconds. DO NOT take your eyes off them. Marshmallow fluff burns fast! Cool and enjoy.